What if every time you had sex, you had to agree to let your partner stab you first — would it be worth the trade-off? While you're mulling that over, consider this: in some species of hermaphrodite sea slug, the trade-off is flipped, with some slugs enduring the unpleasantries of sex in the hope of getting a stab wound.

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Nature — sometimes you can be a scary, scary place.

The sea slugs in question go by the name of Siphopteron quadrispinosum. Mating for these slugs is an incredibly violent affair. A hermaphroditic species, each slug has both male and female sexual organs. Because of this, sex is initiated when one slug uses a syringe-like organ to gore its partner and inject prostate fluid into its body. Then they mate a second time, this time with one slug inserting its penis into the partner's genital opening. These are two separate acts. One is shanking. The other is sexing.

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If sex for sea slugs were like sex for humans, the question for S. quadrispinosum would be the same as the one I posed to you up top, namely: would you sustain a stab wound for sex? But sex for sea slugs is pretty horrifying; to quote evolutionary biologist Rolanda Lange, whose research on S. quadrispinosum sex is published in the latest issue of PLoS ONE:

The penis... bears 4–5 large hook-shaped spines at its base and a crown of 20–30 minute fine pointed spines at its tip, which are spread like an anchor in the female genital tract during mating.

These hooks and spines, as you might expect, are physically harmful when they penetrate another slug during sex. Researchers think this is why slugs prefer to be the ones doing the penetrating, or sometimes avoid mating altogether. Makes sense to us.

But here's the thing: these slugs still allow themselves to be penetrated way more often than necessary. In the course of her investigation, Lange gave pairs of S. quadrispinosum a varying number of opportunities to mate, and found that those which produced the most eggs were the ones who were penetrated more often than was strictly necessary to fertilize said eggs. Why go above and beyond at risk of physically injuring yourself during copulation? New Scientist's Michael Marshall explains:

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In theory, slugs should [allow themselves to be penetrated] just often enough to maintain a store of sperm, and no more. But the slugs [got penetrated] much more often than that. Yet they produced the same proportion of fertilised eggs regardless of how many mating opportunities they had. This indicates that even the slugs that mated the least had gathered more than enough sperm for their reproductive needs.

All of this suggests the traumatic mating has some benefit that goes beyond reproduction – an advantage that offsets the bodily harm. We don't know what that might be, says Mike Siva-Jothy of the University of Sheffield, UK. But the injections of prostate fluid might include nutrients that benefit the stabbed slug.